Introduction

Gas hydrates are naturally occurring

icelike solids in which water molecules

trap gas molecules in a cagelike structure

known as a clathrate. Although many

gases form hydrates in nature, methane

hydrate is by far the most common;

methane is the most abundant natural gas.

The volume of carbon contained in

methane hydrates worldwide is estimated

to be twice the amount contained in all

fossil fuels on Earth, including coal.

Estimates of the global resources of

natural gas hydrate range from 100,000 to

almost 300,000,000 trillion cubic feet

(TCF)—to put these quantities in context,

estimates of the remaining global reserves

and undiscovered resources of conventional natural gas total about 13,000 TCF.

Interest in natural gas hydrates will

increase as finite conventional natural gas

deposits are depleted. As concern increases about the effect of carbon dioxide on

global warming, methane from hydrates

will become an important opportunity.

Combustion of methane adds significantly

less carbon dioxide to the atmosphere than

combustion of either coal or oil. 

Gas hydrates occur at many sites

along continental margins and in the Arctic (fig. 1). Yet we know very little about

their composition, their volume, or the

best ways to exploit their resource potential. The U.S. Geological Survey (USGS)

is investigating many aspects of gas

hydrates to understand their origin, their

occurrence, the factors that affect their stability, and the possibility of using this vast

resource in the world energy mix.

Gas Hydrate: Ice That Burns

Gas hydrate looks very much like

ordinary ice, but if you put a match to it, it

burns with a soft orange flame like the

pilot light on a gas stove (fig. 2). Methane

hydrates have been found in the subsurface in permafrost regions, but most occur

in oceanic sediments hundreds of meters

below the sea floor where water depths are

greater than about 500 meters (fig. 1).

Most occurrences of gas hydrate shown in

figure 1 are inferred from geophysical logs

in the permafrost regions and from anomalous seismic reflectors within ocean sediments called bottom-simulating reflectors,

or BSR’s, which are thought to indicate

the base of the gas hydrate stability zone.

Examination of 90 gas hydrate samples recovered from 15 different geologic

regions indicates that most samples consist

of individual hydrate grains or particles in

pores of sedimentary rocks. In general, the

gas-hydrate-bearing zones in sedimentary

sections range from tens of centimeters to

tens of meters in thickness. Gas hydrate

also occurs as nodules, laminae, and veins

within sediment and, in one case, as a pure

gas hydrate layer as much as 4 meters

thick. 

Vast Quantities, But Are They

Available?

Natural gas hydrate resources appear

to exceed the remaining recoverable conventional natural gas resources of the

Nation and the world. USGS scientists

estimate that the United States has in-place

methane resources in methane hydrates of

Natural Gas Hydrates—Vast Resource, Uncertain Future

U.S. Department of the Interior

U.S. Geological Survey

USGS Fact Sheet FS–021–01

March 2001

Figure 1. Known and inferred natural gas hydrate occurrences in marine (red circles) and permafrost 

(black diamonds) environments. Modified from K. A. Kvenvolden, U.S. Geological Survey (written commun., 

1999). The USGS is studying hydrates at sites 1 (Mackenzie Delta, Canada) and 2 (North Slope, Alaska).

ARCTIC OCEAN

ATLANTIC

OCEAN

PACIFIC

INDIAN OCEAN

OCEAN

INDIAN

OCEAN

2 1

Figure 2. Burning synthetic (manmade)

methane hydrates. Courtesy of the

U.S. Department of Energy.
about 320,000 TCF (statistical mean estimate; see fig. 3); approximately half of

this resource occurs offshore of Alaska,

and most of the remainder is beneath the

continental margins of the lower 48 States.

Estimates of gas hydrate concentrations in

individual accumulations show a considerable range, reflecting our lack of detailed

knowledge. Although the concentration of

methane hydrates in actual accumulations

is still unclear, the possibility exists that

large, economically extractable concentrations of energy exist in methane hydrate

reservoirs.

USGS scientists have been conducting

research on gas hydrates to answer questions like the following:

• In what form do gas hydrates occur in

sediments?

• What is the quantity of gas actually

available in individual gas hydrate

accumulations?

• What production technology is

required to develop this resource in a

cost-effective manner?

While we don’t have all the answers

yet, we have made progress in understanding the challenges of recovering the gas

from the hydrate. Proposed methods usually focus on dissociating or “melting” inplace gas hydrates by three methods:

• Heating the reservoir above hydrate

stability temperatures by using hot

water or steam injection

• Injecting an inhibitor, such as

methanol or glycol, to decrease

hydrate stability

• Decreasing reservoir pressure below

hydrate equilibrium, thus allowing

release of the methane gas contained

in the clathrate

Although thermal stimulation is technically possible, the economic cost would

be prohibitive. Similarly, flooding the

hydrate accumulation with inhibitors

appears to be technically feasible but

impractical both economically and environmentally. The most economically

promising method of producing gas from

methane hydrates thus appears to be

depressurization.

There are serious technical challenges

to overcome. Most known marine gas

hydrates are in fine-grained, clay-rich sediments with little or no permeability. Flow

of gas to a well bore requires effective

paths of fluid flow, but establishing such

paths in low-permeability sediments may

be exceedingly difficult. Where porous

coarse-grained sediments occur on the

continental margins within the zone of

methane hydrate stability, such as in the

Gulf of Mexico and off the eastern coast

of India, fluid flow may be more easily

established.

Hazards

Gas hydrates represent a hazard to

conventional oil and gas production. In the

Gulf of Mexico, oil and gas exploration is

extending into water depths where gas

hydrates occur at the sea floor. Pumping

hot oil from great depths through drill

pipes can cause warming of sediments and

dissociation of hydrate, liberating large

amounts of methane, weakening sediments, and perhaps generating pockets of

highly pressured gas. The result might be

gas blowouts, loss of support for pipelines,

and sea-floor failure that could lead to

underwater landslides and the release of

methane from hydrates.

Current USGS Research

The USGS is investigating the

resource potential of gas hydrates in two

locations in the Arctic (fig. 1): the

Mackenzie Delta of Canada and the North

Slope of Alaska. In Canada, the USGS is

working with an international consortium,

led by the Geological Survey of Canada,

to test technology to economically produce

gas hydrates. In Alaska, the USGS, in partnership with the U.S. Department of Energy, is assessing the recoverability and

potential production characteristics of

onshore natural gas hydrate accumulations

overlying the Prudhoe Bay and Kuparuk

River oil fields. It is estimated that the

known gas hydrate accumulations in

northern Alaska may contain as much as

45 TCF of natural gas. 

The Bottom Line

The immense volume of methane

hydrates worldwide may be a potential

resource of extraordinary richness. Our

understanding of these resources, however, is rudimentary—we do not yet know if

these accumulations exist in sufficient

concentration to make them economically

viable, nor do we know enough about

production technology to assume that

even concentrated accumulations can be

developed economically.

Use of natural gas will expand far into

the 21st century due to increasing demand

for cleaner burning fuels and greater use in

the transportation sector. It may well be

that government policy, responding, for

example, to growing concern about climate

change, will also influence the development of methane hydrates. In the past, government subsidies have driven development of other unconventional natural gas

resources, including coal-bed methane.

Now, as technology has developed, coalbed methane is a viable fuel in its own

right, free of governmental incentives. 

Despite all the apparent obstacles to

the development of gas hydrate resources,

it is good to remember that extraordinary

technological developments in the petroleum industry—three-dimensional seismic

techniques, secondary recovery methods,

and horizontal drilling, for example—have

led to the exploitation of resources once

thought to be unavailable. Natural gas

hydrates may also become economically

extractable.

For more information, please 

contact:

Timothy Collett

U.S. Geological Survey

Denver Federal Center, MS 939

P.O. Box 25046

Denver, CO 80225

Telephone: 303–236–5731

E-mail: tcollett@usgs.gov

0 200,000 600,000

90

80

70

60

50

40

30

20

10

0

100

PROBABILITY OF MORE THAN

ESTIMATES

Mean = 9,069 TCM

F95 = 3,193 TCM

F5 = 19,147 TCM

TCF

IN-PLACE METHANE

0 10,000 20,000 TCM

Figure 3. Cumulative probability curve showing the estimated in-place methane resources

within the methane hydrates of the United

States. TCF, trillion cubic feet; TCM, trillion

cubic meters. The curve is read as follows:

there is a 95 percent chance (F95) that the

methane hydrate resource potential is greater

than 112,765 TCF (3,193 TCM), and there is a 5

percent chance (F5

) that the resource is

greater than 676,110 TCF (19,147 TCM). The

mean estimate is 320,222 TCF (9,069 TCM).

Modified from figure 17 of T.S. Collett’s article,

Gas hydrate resources of the United States, in

U.S. Geological Survey Digital Data Series

DDS–30, release 2, 1996.